Generally speaking, presently available protection circuits such as earth leakage circuit breakers (ELCB's) operate quite satisfactorily in detecting faults such as ground faults and overloads. Other faults which may occur are opening of the line neutral or ground leads. In the first mentioned situation there is little hazard present, however, in the case of loss of the neutral connection the line voltage still remains. Loss of ground is clearly also quite hazardous.
In an attempt to overcome this problem, protection circuits have been devised which provide loss of neutral or ground protection. For example, U.S. Pat. No. 4,598,331 (Legatti) describes such a circuit. The Legatti protection circuit includes a differential transformer with a pair of opposed primary windings and a secondary winding for detection of ground faults and an additional supplemental winding between the neutral and ground leads for detection of open neutral or ground connections. Another example of a protection circuit which provides loss of neutral or ground protection is that described in European Patent Specification No. 154,450 (Delta). This circuit has a circuit breaker with two SCR's, one for tripping on detection of a ground fault, and a second for tripping on detection of loss of neutral or ground.
While these circuits appear to be effective, they suffer from the disadvantage of being relatively complex. In the Legatti circuit, for example, a winding, resistors and diodes are required, whereas in the Delta circuit a separate SCR is required. Further, in the prior art, disconnection of the line lead is instantaneous on detection of loss of neutral, which is not satisfactory when continuity of operation of electrical equipment is required when loss of neutral is only momentary.